1. Field of Invention
The present invention relates to a heating device used in the production process for semiconductors and thin-film transistors, and relates to a flash lamp heating device using a flash lamp as a heating source.
2. Description of Related Art
Conventionally, in order to inject ions onto a most-surface of a substrate, such as a semiconductor wafer, or to activate a substrate, the substrate is rapidly heated, and consequently, a device for heating a substrate using a flash lamp is well known (see, Japanese Laid-Open Patent Application No. 2002-198322 and Japanese Laid-Open Patent Application No. 2001-319887 (corresponding to US 2002-0179589 A)).
Further, a device for heating a substrate by light from both surfaces, wherein background heating (preheating) is conducted using a halogen lamp, and then, the substrate is rapidly heated with a flash lamp to the temperature for activating the substrate is also well known (see, International Patent Application Publication No. WO 03/085343).
The flash lamp is a lamp where luminescent gas, for example, xenon (Xe), is enclosed in the sealed inside of a rod-shaped luminous tube, for example, made from silica glass, and a pair of electrodes are arranged by facing each other inside the rod-shaped luminous tube. A rod-shaped conductor, for example, made from stainless steel is placed on the outer surface of the luminous tube of the flash lamp along a longitudinal direction of the luminous tube as a trigger electrode. The flash lamp is lit by supplying high voltage to the trigger electrode.
FIG. 9 shows an example of a lighting circuit of the conventional flash lamp.
A coil 23 is connected to a high-voltage side 22 of a flash lamp 5 and to ground 24, and a capacitor 26 is connected in parallel to a series circuit of the flash lamp 5 and the coil 23. Energy is supplied to the flash lamp from the capacitor 26. Supplying energy to the capacitor 26 is started by switching on a switch SW1 arranged at the high-voltage side 22.
In addition, a trigger electrode 52 is placed to illuminate the flash lamp 5, and the trigger electrode 52 is connected to a trigger coil 30. Switching on a switch SW2 and supplying voltage pulses HV to the primary side of the trigger coil 30 cause the application of high voltage to the trigger electrode 52, and the flash lamp 5 is lit.
Lower power, more compact semiconductor integrated circuits have made the transistor circuit produced within the circuit itself to become a very micro-fabricated circuit. Specifically, it is necessary to reduce the depth of a diffusion layer of impurity atoms contained in the semiconductor layer for forming the source and drain at both sides of the gate in the transistor circuit. On the other hand, the surface resistance value (Ω/cm2) of the semiconductor circuit needs to be lowered.
The depth of the diffusion layer of the impurity atoms in the transistor circuit formed on the semiconductor wafer can be reduced by lowering the diffusion temperature or shortening the time during doping of impurity atoms in a diffusion process to dope and diffuse impurity atoms on the semiconductor wafer.
On the other hand, in the activation process to activate the impurity diffusion layer and to lower the surface resistance value (Ω/cm2), impurities (dopant) to be diffused on the semiconductor wafer are positioned not in alignment to the silicon crystal lattice location after the diffusion process; however, the activation is completed when the dopant itself finds the closest crystal lattice and returns to a proper position. This phenomenon requires only such a short time as approximately 10 nano-seconds.
Achieving both the high activation and the lower diffusion is realized by rising the temperature as much as possible and conducting a thermal treatment in a short time.
For example, if the material used for the semiconductor wafer is silicon, it should be heated at around 1,400° C., which is a temperature sufficient to melt silicon, for a moment.
As one example, a case of a silicon wafer using boron as a dopant is shown. The silicon wafer needs to be heated at 1,000° C. or higher for 1.5 seconds or longer, when the silicon wafer is heated by the conventional spike RTA (optical rapid thermal annealing using a halogen lamp), and setting the resistance value to 1,000 Ω/cm2. However, heating at this temperature for this time period will move (diffuse) boron at a certain concentration, which is situated around 10 nm deep before heating, to a depth around 30 nm after the spike RTA heating.
On the other hand, if heating with a flash lamp and similarly irradiating so as to bring the resistance value to 1,000 Ω/cm2, boron at a certain concentration situated around 10 nm deep does not excessively diffuse in the depth direction even after heating with the flash lamp, and stays around 10 nm. Heating for a short period is necessary in order not to diffuse the dopant, and heating with the flash lamp makes this possible in actuality.
In fact, when the heating time becomes longer and the temperature throughout the entire semiconductor wafer rises higher, the dopant diffuses in the depth direction of the semiconductor wafer. However, heating with the flash lamp can prevent excessive ion diffusion.
However, when actually heating a substrate, such as semiconductor wafers, using a flash lamp, the temperature of the substrate rises abruptly by light irradiation, and by the abrupt rise of temperature, there are problems of deformation or cracking by the thermal strain occurring to the substrate.
As mentioned above, using a flash lamp enables short-time heating without ion diffusion spreading the entire substrate. However, rising the temperature abruptly for a short time causes problems, such as deformation or cracking due to the thermal strain resulting from the difference in temperature on the surface and the bottom of the substrate.